Downhole tools for earth boring and for other purposes, including rotary drill bits, are commonly used in bore holes or wells in earth formations. One type of rotary drill bit is the roller cone bit (often referred to as a rock bit), which typically includes a plurality of conical cutting structures (often referred to as cones or cutters) secured to legs dependent from the bit body. For example, the bit body of a roller cone bit may have three depending legs each having a bearing pin (otherwise referred to as a journal pin). A rotatable cone may be mounted on each of the bearing pins. The bit body also may include a threaded upper end for connecting the drill bit to a drill string. During drilling, the rotation of the drill string produces rotation of each cone about an associated bearing pin thereby causing the protruding elements on the cone, which may be integrally formed with the cone or comprise inserts secured to the cone, to engage and disintegrate the rock by a crushing and grinding action.
The bearing surfaces employed between the cones and the bearing pin are often the source of significant operational problems during drilling, as these bearings operate in an extremely hostile environment due to high and uneven loads, and elevated temperatures and pressures. Particulate matter present in both the cuttings from a formation being drilled and the solids-laden drilling fluid often enter into the gap between cooperating bearing surfaces, causing accelerated wear. This is particularly true when drilling deep bore holes under high pressures. In addition, rock bits are subject to corrosive chemical environments, again from both the formation environment and chemicals employed in drilling fluids. Another factor that can lead to early bearing failure is the inability of the bearings to withstand changes in the magnitude of forces directed against the roller cone. For example, the side forces (e.g., applied from the side of the bore hole) may tend to deflect the cone off its designed axis of rotation, pinching the bearings and contributing to early bearing failure. In addition, as the bearings wear and gaps between cooperating bearing surfaces increase, more wobble of the cones on the bearing pins may occur. The resulting play in the bearing assembly increases the wear rate on the bearing elements as well as the sealing elements in the cone intended to prevent intrusion of well bore fluids, limiting the usable life of the bit. In addition, the limits of the bearing's capacity may limit both the load that can be applied to the bit as well as the angular velocity at which the bit can be rotated, each of which constrains achievable penetration rates and feasible cutter designs.
In order to withstand the extremely hostile environment, bearings may be formed from a variety of wear-resistant materials. However, further difficulties may arise in coupling or integrating such wear-resistant bearings with the other components of downhole tools in a desirable and reliable manner.